The present invention relates to a vibration detecting sensor using piezo-electric elements, and more particularly to a vibration detecting sensor capable of eliminating the effects of temperature and noise.
Generally, a vibration detecting sensor is used in an electronic sphygmomanometer and also used for detecting vibrations in a machine.
Referring to FIGS. 1 through 3, a conventional vibration detecting sensor is comprised of a circular vibrating plate 1, a covering member 2 covering the circular vibrating plate 1, a piezo-electric element 3 attached to the center of the upper surface of the vibrating plate 1, and a lead-in wire 4 connected electrically to the piezo-electric element 3.
In the conventional vibration detecting sensor, the vibrating plate 1 is composed of a material such as plastics or rubber having metallic particles distributed therein. The piezo-electric element 3 made of a complex material composed of a rubber or a synthetic resin having ferro-electric particles such as PZT or BaTiO.sub.2 distributed therein, is attached to the vibration plate 1 by an adhesive. The covering member 2 composed of a metallic material includes a cylindrical wall 2a and a circular ceiling 2b on top of the wall 2a. At the bottom of the wall 2b, there is a channel-shaped section 2c such that the vibrating plate 1 snaps into the channel shaped section 2c and combines with the covering member 2. Also, a slot 2d is formed through the wall 2a and the channel-shaped section 2c so that the lead-in wire 4 can be inserted within the covering member 2. An end of a central wire 4a in the inserted lead-in wire 4 is soldered onto the upper surface of the piezo-electric element 3 by a soldering portion 5, and the shielding wire 4b of the lead-in wire 4 is connected to the wall 2a, also by soldering.
When the vibrating plate 1 of the above-described conventional vibration detecting sensor is placed into contact near an arterial blood vessel of a person or a machine, the vibrating plate 1 vibrates according to the pulses of the person or the vibrations of the machine, and to thereby an electrical potential difference between the upper and lower surface of the piezo-electric element 3. The vibrating plate 1 electrically conducts through the metallic particles distributed therein, and the shielding wire 4b connected to the wall 2a of the covering member 2 is also connected electrically to the bottom side of the piezo-electric element 3. Thus, electrical potential differences generated on the piezo-electric element 3 is output as an electric signal via the lead-in wire 4, and thus vibrations can be detected.
However, because the above-described conventional vibration detecting sensor uses a single-plate piezo-electric element, there are problems in that error signals are generated due to temperature changes of its surroundings. Also, because the lead-in wire 4 is inserted into the covering member 2 through the slot 2d on the slot 2a, external noises can penetrate through the hole 2d.